Solid State Drives for Energy Savings
Jim Handy (Objective Analysis) and Tom Coughlin (Coughlin Associates) on behalf of SNIA Solid State Storage Initiative

Enterprise flash solid state drives (SSDs) have recently caught the eye of the data center manager. These high-speed devices, that can boast read IOPS rates two orders of magnitude higher than enterprise HDDs, are typically used to increase data transfer rates. SSDs offer new options for enhancing the performance of data centers. Although more expensive than HDDs from the perspective of $/GB, solid state storage has been shown to provide a cost advantage over HDDs when measured in $/IOPS, and power consumption, especially for read-intensive applications.

Once the data center manager incorporates SSD technology to complement existing storage tiering, two important side benefits of SSD storage become apparent: SSDs often consume less power than HDDs, and an equivalent I/O bandwidth usually consumes significantly less floor space in the data center. Thus SSDs and other solid state storage technology complement the lower $/GB of HDD array storage by providing a lower $/IOPS and operating cost. As a consequence a complete storage system incorporating both storage technologies can provide tiered storage with both higher performance and cost effective mass storage.

Both of these side benefits carry significant implications. Data centers have two inviolate limits: their power budget and their floor space. Once computing requirements expand beyond either of these, management must embark on the construction of a new data center at considerable expense. Since SSDs can squeeze more I/O performance into a smaller space at lower power they become a welcome means of increasing data center throughput within a constrained budget.

Furthermore, the lower power consumption of an SSD benefits the data center in two ways. Most data center managers use a rule of thumb that every Watt of power consumed by a device must be matched by anywhere from one to two additional Watts used to cool that dissipated power. Any change that can provide the same computing horsepower using less energy will reap double or triple the energy savings, even further stretching the data center’s power budget. Solid state storage combined with hard disk drive storage can result in lower overall operating costs through the reduction of data center energy costs.

Total Cost of Ownership (TCO)

The promise of power savings is so significant that it is an important component in the Storage Network Industry Association (SNIA) Solid State Storage TCO calculator, which can be downloaded for free at www.SNIA.org. The SNIA TCO calculator estimates the comparative costs of complementing an HDD-based high-speed storage system with either SSD storage or higher-speed HDDs and outputs estimates of hardware, energy, repair, and cabinetry costs. The TCO calculator takes into account the drive power plus the enclosure power needed to support the drives. The model does not include estimate of certain very difficult metrics like reduced data center floor space consumption which could amplify cost savings.

Comparing SSD and HD Power

A flash SSD typically uses less power than a hard disk drive, and SSDs can often replace an array of HDDs that have been configured to achieve an extreme level of performance. This is particularly true in cases where a system’s storage capacity requirements are small but the I/O demand is high. The performance level of an SSD may match or surpass that of an HDD array configured to provide high IOPS performance by sacrificing storage capacity.

Total power consumption is a difficult parameter to measure, though, since HDDs consume varying amounts of power, depending on whether the hard disks are already rotating at full speed or must be spun up, whether the head is being moved a significant distance, and whether there are many cache hits to the internal DRAM cache.

Enterprise HDDs achieve high throughput by combining faster spindle speeds with faster access times but an enterprise HDD consumes more power than an enterprise SSD.

To further complicate things, HDD-based systems that need even greater performance may employ short stroking. Short-stroking is an approach system designers use to coax the highest performance out of an HDD array by only using the outer radii of the disks. A short-stroked HDD array can consume a significant amount of power while sacrificing much of the otherwise usable storage capacity.

Another way to improve a storage system’s IOPS is to stripe data across a number of drives to create parallel data paths through which multiple simultaneous data streams can be written and read. Either of these two approaches – short-stroking or striping – multiply the number of HDDs in the system, increasing power consumption.

As is the case with an HDD, the operating power of an SSD also breaks down to operational components: Read power, write power, and idle power. An HDD’s idle power is not that different from the read power of an SSD, and an SSD’s write power is significantly higher than its read power.

The only way to accurately compare the power consumption of an HDD and an SSD is to present them in the same operating environment and compare power consumption in that environment. This is rarely feasible, so it is useful to try and get a rough idea of the outcome by modeling performance in a system.

Table 1, was derived by the SNIA SSD TCO calculator and appears in a recent Objective Analysis Report: Solid State Drives in the Enterprise, Update 2010. Data from manufacturers’ specifications for two SSDs and five HDDs were run through the calculator. The data in Table 1 represents the drive power plus the fraction of the enclosure power used per drive to calculate the power per IOPS. The power consumed by all of the devices is relatively narrowly distributed between 6.5-24.3W, but since the IOPS figures for the SSDs are orders of magnitude higher than those of any of the HDDs, the power consumption per IOPS for the SSDs are significantly lower than those of any of the HDDs.

Table 1. Comparing the Power Consumed by an Enterprise SSD and three HDDs

The rightmost column clearly shows that substantial power savings can be realized in high IOPS systems by using an SSD to replace one or more HDDs.

Let’s zero in on the IOPS column, though. The huge IOPS difference between the SSDs and the HDDs means that a single SSD can often replace more than one HDD if the actual storage capacity required is smaller than the capacity of the SSD. We mentioned before that high-performance enterprise systems today sometimes use an array of short-stroked high performance HDDs to achieve the system’s throughput goals sacrificing capacity for throughput. SSDs can often meet or exceed the throughput of such short-stroked arrays and match their modest storage capacity at a cost that is equivalent or better than that of the short-stroked HDD array.

Examples of applications that could benefit from the use of an SSD rather than an HDD array would be data base access and metadata management where the stored content is accessed frequently, written much less often than read, and requires a modest storage capacity.

Individual SSDs consume less power than individual HDDs and will certainly use less power than an array of high performance HDDs in high IOPS, low storage capacity applications. If the storage capacity for the application can be handled by one or a few SSDs then the operating cost savings realized by replacing ten or twenty HDDs with a small number of SSDs will be significant. Even if the original purchase price of an SSD is significantly higher than that of an enterprise HDD, the total cost of ownership might be lower, since the TCO depends not only upon the storage capacity requirements but also upon operating costs, including power consumption as well as cooling costs.

In these days of energy consciousness the IOPS per Watt argument carries a lot of sway in the decision to complement an HDD storage system with an SSD, even if the price per IOPS argument is marginal.

For more information about SNIA’s Solid State Storage, SNIA Europe, please visit http://www.snia-europe.org/solidstate/ or SNIA North America, http://www.snia.org/forums/sssi/

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